Many industrial and consumer products rely on the liquid and vapor permeation property of materials to attain desired effects. For example, residential and commercial construction techniques typically include the installation of a vapor barrier to prevent loss of insulative properties of thermal insulation materials. Mechanical and electronic components are often coated with conformal polymeric materials to prevent corrosion. Food products such as cereals and snack products are typically packaged in materials designed to prevent infusion of humidity which reduces crispness. In these applications materials are designed and used to substantially prevent the permeation of fluids in and/or out. However, there are many other industrial and consumer applications that benefit from allowing some permeation of vapors through a material. For example, industrial polymeric membranes are designed to permit the transfer of permeates through their surface. Fabrics for personal outerwear are often treated with polymers for rain resistance, but the removal through the fabric of water vapor generated by the wearer is beneficial to enhance comfort. Contact lenses are designed for gas permeability.
The above examples illustrate the benefit of controlling the permeation of water and other fluids through material surfaces within a range of values. Achieving that objective requires the accurate measurement of permeation rates of vapors through the materials. The ASTM E96-00, “Standard Test Methods for Water Vapor Transmission of Materials,” is a standard test method for measuring the water vapor permeability of materials. It is used primarily for construction materials and fabrics to measure the effect that water has on a particular material or product. The desiccant (or “dry cup method”) and the water (or “wet cup method”) are the two basic methods used for ASTM E96 tests. A variation of the test uses a combination of the two methods where a test compartment is divided into two chambers separated by the material being tested and a moist atmosphere is established in one chamber and a dry atmosphere is in the other. A test is run to determine how much moisture passes through the test material from the “wet” chamber to the “dry” chamber.
Water Vapor Transport is calculated using the formula presented as Equation 1:WVT=(G/t)/A  [Eq'n 1]where G is the weight gain of the desiccant, t is time (typically twenty four hours under ASTM E96), and A is surface area of the material between the chambers. Typically the quantity of G/t is found by plotting the water weight gain or loss versus time and obtaining the slope of that line.
Permeance is calculated from the formula presented as Equation 2:Permeance=WVT/(S¤(R1−R2))=WVT/ΔP=G/(t¤A¤ΔP)  [Eq'n 2]where S is the saturation vapor pressure at test temperature, R1 is the relative humidity at the vapor source, and R2 is the relative humidity at the vapor sink (such as at a desiccant), and ΔP is the vapor pressure differential. Saturation vapor pressure is the pressure that water vapor creates when the air is fully saturated. When air is at saturation vapor pressure, it is said to be at the dew point. In the dry cup method, the relative humidity inside the test cup is controlled to be approximately 0% and the relative humidity outside of the test cup is generally controlled at a high value, typically 95%-100%. In the wet cup method, the relative humidity inside the test cup is set at a high value, typically 95%-100%, and the humidity outside the cup is controlled to be approximately 0%.
ASTM E96 water vapor transmission measurements are often difficult to make. Sometimes the results obtained by the dry cup method do not agree with results obtained by the wet cup method. These discrepancies are generally attributed to the dominance of factors related to vapor outflow from the test specimen in the dry cup method and dominance of factors related to vapor flow into the test specimen in the wet cup method. Another difficulty is that these tests are typically limited to materials of a particular form such as sheets or films. What is needed, therefore, are mechanisms and methods for more directly and accurately measuring the permeation rate of polymeric and other materials in various forms, including sheets, tubes, and other structures.